10. Other miscellaneous Organic Redox Reactions

This is a 'collection' of reactions not dealt with in sections 8. and 9. They may/may not be useful reactions.

Section 10. reaction sub–index: 10.1 Cannizzaro reaction * 10.2  aldehydes/ketones tests * 10.3 Combustion * 10.4 Fuel cells * 10.5 The use of 1,4–dihydroxybenzene (quinol, hydroquinol) in photography

• 10.1 The Cannizzaro reaction

  • Aldehydes which do not have a hydrogen atom on the carbon next to the carbon of the carbonyl group (C=O) undergo the Cannizzaro reaction with concentrated aqueous sodium hydroxide in which one molecule of the aldehyde is reduced to a primary alcohol and another is oxidised to the sodium salt of a carboxylic acid.

  • This is an organic example of disproportionation in which the same carbon atoms of the reactant molecule simultaneously increase and decrease their oxidation state e.g.

    • methanal changes to methanol and sodium methanoate

      • 2HCHO + Na⁺OH^– ==> CH₃OH + HCOO^–Na⁺

    • benzaldehyde changes to phenylmethanol (benzyl alcohol) and sodium benzoate

      • 2C₆H₅CHO + Na⁺OH^– ==> C₆H₅CH₂OH + C₆H₅COO^–Na⁺

• 10.2 Simple chemical tests to distinguish aldehydes from ketones

  • The tests depend on the relative redox stability of ketones compared to the much more readily oxidised aldehydes. These tests also give positive results with many reducing sugars and some rather more stable aromatic aldehydes e.g. benzaldehyde, may not give a positive result at all.

  • In these tests, because aldehydes are stronger reducing agents than ketones, they reduce the metal ion and are oxidised in the process i.e. RCHO + [O] ==> RCOOH, a change which is equivalent to a 2 electron loss by the RCHO.

  • Tollens reagent is a colourless solution of silver nitrate in aqueous ammonia.

    • When an aldehyde is warmed with Tollens reagent it is oxidised to a carboxylic acid and the silver ion (in ammine complex form) is reduced to silver, forming a silver mirror on the side of the test tube.

    • 2[Ag(NH₃)₂]⁺_(aq) + R–CHO_(aq) + H₂O_(l)  ==> 2Ag_(s) + 4NH_3(aq) + R–COOH_(aq) + 2H⁺_(aq)

    • simplified: 2Ag⁺_(aq) + R–CHO_(aq) + H₂O_(l)  ==> 2Ag_(s) + R–COOH_(aq) + 2H⁺_(aq)

    • or 2[Ag(NH₃)₂]⁺_(aq) + R–CHO_(aq) + 2OH^–_(aq) ==> 2Ag_(s) + 4NH_3(aq) + R–COOH_(aq) + H₂O_(l)

    • simplified: 2Ag⁺_(aq) + R–CHO_(aq) + 2OH^–_(aq) ==> 2Ag_(s) + R–COOH_(aq) + H₂O_(l)

    • Ketones show no reaction because of their greater stability to oxidation.

  • Fehlings or Benedict's solution consists of a copper(II) ion complexed with an organic carboxylic acid.

    • The deep blue copper(II) ion in the complex is reduced to a red–brown precipitate of copper(I) oxide.

    • Ketones show no reaction due to their greater oxidation stability.

    • 2Cu²⁺_(complex/aq) +  R–CHO_(aq) + 2H₂O_(l) ==> Cu₂O_(s) + R–COOH_(aq) + 4H⁺_(aq)

    • or: 2Cu²⁺_(complex/aq) +  R–CHO_(aq) + 4OH^–_(aq) ==> Cu₂O_(s) + R–COOH_(aq) + 2H₂O_(l)

    • Ketones show no reaction because of their greater reluctance to oxidation.

  • There are lots more organic chemical tests described – Chemical Identification Tests (with alphabetical index).

• 10.3 All organic compound air/oxygen combustion reactions are oxidations in terms of the carbon atoms of the organic molecule e.g. carbon's oxidation state is increased from (–4) in methane to (+4) in CO₂ for complete combustion and (+2) if carbon monoxide formed or (0) in carbon (soot) if the combustion is inefficient/incomplete. In each case oxygen's oxidation state changes from (0) to (–2) to offset the increase in carbon's oxidation state. e.g.

  • CH_4(g) + 2O_2(g) ==> CO_2(g) + 2H₂O_(l)

  • 2CH_4(g) + 3O_2(g) ==> 2CO_(g) + 4H₂O_(l)

  • CH_4(g) + O_2(g) ==> C_(s) + 2H₂O_(l)

• 10.4 Organic Fuel Cells

  • Many hydrocarbon molecules are burned as fuels, but since it is a redox reaction, theoretically it can be done as a combined half–cell oxidation/reduction electron transfer reaction.

  • In practice organic compounds can be oxidised by oxygen in a way that can be used to generate electricity directly in a fuel cell (right diagram) rather than release the energy as heat e.g.

  • In reaction 9.1(a) ethanol was oxidised to ethanoic acid by acidified potassium dichromate(VI). The same result can be obtained by reaction of ethanol with oxygen in a fuel cell.

  • The reaction is very exothermic, so ethanol is a good source of chemical potential energy.

  • Ethanol is used directly as a fuel in a direct ethanol fuel cell (a DEFC fuel cell).

  • (i) CH₃CH₂OH_(aq) + O_2(g) ==> CH₃COOH_(aq) + H₂O_(l) (ΔH^Ø = –494 kJ mol^–1)

  • The half–cell reactions are:

    • (ii) Reduction, +ve electrode: O_2(g) + 4H⁺_(aq) + 4e^– ==> 2H₂O_(l) (E^Ø = +1.23V)

    • (iii) Ox'n, –ve electrode: CH₃CH₂OH_(l) + H₂O_(l) ==> CH₃COOH_(aq) + 4H⁺_(aq) + 4e^– (E^Ø = +0.06V)

    • Adding (ii) + (iii) = equation (i) and E^Ø_cell = E^Ø_+/red'n – E^Ø_–/ox'n = 1.23 – 0.06 = +1.17V

    • The reactions must take place on catalytic electrodes made of platinum and other transition metals and here the inner 'electrolyte' is as a polymer proton exchange membrane of the fuel cell (a PEFC fuel cell), though it can be a concentrated phosphoric acid solution (in a PAFC fuel cell).

    • The electrons will flow through the external circuit from the –ve electrode to the +ve electrode.

    • Free energy change: ΔG^Ø = –nE^ØF = – 4 x 1.17 x 96500 = –451620 J mol^–1 = –451.6 kJ mol^–1

    • n = number of electrons transferred,  E^Ø = cell voltage, F = Faraday constant in coulombs mol^–1 

    • This sort of chemistry is being developed to make reasonably efficient portable fuel cells

  • Commercially developed fuel cells will hopefully convert the ethanol completely into water and carbon dioxide to give a greater and more efficient energy output, but its still generates a voltage of 1.0–1.2V per cell, in this case ...

    • (i) Oxidation, –ve electrode: C₂H₅OH_(l) + 3H₂O_(l) ==> 12H⁺_(aq) + 2CO_2(aq/g) + 12e^–

    • (iii) Reduction, +ve electrode: 3O_2(aq/g) + 12H⁺_(aq) + 12e^– ==> 6H₂O_(l)

    • adding (i) + (iii) gives: C₂H₅OH_(l) + 3O_2(aq/g) ==> 3H₂O_(l) + 2CO_2(aq/g)

    • + other organic products as the reaction is not completely efficient.

    • The cells can obviously be connected in series to give larger voltages.

    • The ethanol ('alcohol', C₂H₅OH) can be bio–sourced from sugar beet, potatoes and cereal crops and other plant material that can fermented with enzymes. The ethanol is fractionally distilled from the fermented mixture and constitutes a renewable fuel.

  • However there seems to have been more work done? on the Direct Methanol Fuel Cell (a DMFC fuel cell), though there are concerns over methanol's toxicity and the very costly platinum catalytic electrodes  required, but the DMFC is essentially like the DEFC described above and the principles illustrated in the diagram.

  • The methanol can be synthesised in the reforming reaction CO_(g) + 2H_2(g) ==> CH₃OH_(l)

    • half–cell reactions for the DMFC cell:

      • (i) Red'n, +ve electrode: ³/₂O_2(g) + 6H⁺_(aq) + 6e^– ==> 3H₂O_(l) (E^Ø = +1.23V)

      • (ii) Ox'n, –ve electrode: CH₃OH_(aq) + H₂O_(l) ==> CO_2(aq/g) + 6H⁺_(aq) + 6e^– (E^Ø = ?V)

      • so overall the cell reaction is ... with a maximum output voltage of about 1.0V.

      • CH₃OH_(aq) + ³/₂O_2(g) ==> CO_2(aq/g) + 3H₂O_(l)

  • Hopefully, efficient cells using ethanol can be developed because ethanol compared to methanol has a higher energy density (e.g. kJ/kg), is less toxic and can be bio–resourced as a renewable fuel.

  • There is a huge amount of research going on in fuel cell development to try to use cheaper, but equally effective tiny particle metal catalysts of Fe, Co and Ni instead of costly platinum, but the most efficient metals are still the most costly?

• 10.5 The use of 1,4–dihydroxybenzene (p–quinol, hydroquinol) in photography

  • 1,4–dihydroxybenzene (Quinol) is an ingredient in photographic developing solutions. In the transparent plastic emulsion film of the exposed film it will reduce the silver ions in silver halide salts, (which have not already been decomposed by light to silver and bromine), to silver as well. It would appear that these extra silver atoms produced by the developer, cluster around the already formed silver atoms (acting as 'nuclei') from the action of light, to enhance the image in 'shades' and 'contrast' to give a more clearly defined negative.

    • The relevant half–cell reactions and their standard redox electrode potentials are ...

    • AgBr_(s) + e^– ==> Ag_(s) + Br^–_(aq)  (E^θ_Ag/AgBr = +0.073V at 298K)

    • _(aq) + 2H⁺_(aq) + 2e^– ==> _(aq)  (E^θ_Q/QH2 = +0.059V at 298K, pH 8.5)

    • These two organic molecules are referred to by a variety of systematic and trivial names, confusing when searching the web! e.g.

      • left (ref Q): 2,5–cyclohexadiene–1,4–dione, p–quinone,  ('one' higher ranking than 'ene')

      • right (ref QH2): 1,4–dihydroxybenzene, 1,4–benzenediol, benzene–1,4–diol, p–quinol, p–hydroquinone, p–dihydroxybenzene

  • It is convention to show the half–cell reactions as reductions when quoting them with the half–cell potential.

  • In the reactions the silver ions in the silver bromide salt are reduced to silver atoms and 1,4–dihydroxybenzene (quinol) is oxidised to 2,5–cyclohexadiene–1,4–dione (quinone). The bromide ions 'dissolve' and two hydrogen ions are released from the quinol. Therefore the basic reaction via the alkaline developer is ....

    • _(aq) + 2AgBr_(s)  ==> _(aq) + 2Ag_(s) + 2Br^–_(aq) + 2H⁺_(aq)

    • Sometimes more simply written as ...

    • C₆H₄(OH)_2(aq) + 2AgBr_(s) ==> C₆H₄O_2(aq) + 2Ag_(s) + 2HBr_(aq)

    • E^θ_reaction = E^θ_red – E^θ_ox = E^θ_AgBr/Ag – E^θ_Q/QH2

    • E^θ_reaction = (+0.073) – (+0.059) = +0.014V

    • The positive value for E^θ_reaction shows the reaction is feasible. In fact the E^θ_Q/QH2 becomes less positive as the pH is increased making the reaction more feasible, but the E_AgBr/Ag is theoretically independent of pH. The hydrogen ions would be neutralised by the alkaline media.

  • Oxidation number analysis for the molecules in question is shown below ...

    • using graphic structural formulae and

    • Oxidation states in organic compounds is explained in section 11. below.

    • 

  • Note: Not all the silver bromide reacts in this way, so in the fixing process the remaining silver bromide is removed by sodium thiosulphate to produce the 'light stable' negative.

  • The transition metal complex ion [Ag(S₂O₃)₂]^3–_(aq) is formed when sodium thiosulphate (Na₂S₂O₃) is used to remove unreacted silver bromide (AgBr) crystals in developing photographic films.

    • AgBr_(s) + 2S₂O₃^2–_(aq) ==> [Ag(S₂O₃)₂]^3–_(aq) + Br^–_(aq)

    • This is NOT a redox reaction, Ag is +1 and Br is –1 throughout the reaction. The thiosulfate ion is here acting as a ligand and not a reducing agent e.g. like its reaction with iodine.

• –

11. Oxidation state and organic compounds

Usually the oxidation state of hydrogen is +1, and oxygen –2 in organic compounds.

(The quoted Pauling electronegativities are C = 2.5, H = 2.2 and O = 3.5 which gives the lead in assigning oxidation numbers in this section i.e. the highest oxidation state is assigned to the least electronegative atom and vice versa for hydrogen and oxygen BUT beware for carbon, logical deduction can give some surprising, but correct results!)

On this basis you can achieve a useful oxidation number analysis of simple organic compounds in an oxidation sequence.

e.g. the oxidation sequence below, with the oxidation state of carbon in () and in hydrogen in ().

CH₄ (–4) == ox'n ==> CH₃OH (–2) == ox'n ==> HCHO (0) == ox'n ==> HCOOH (+2) ==> CO₂ (+4)

The above sequence can described in terms of the 'level of the functional group' which is equal the number of bonds the carbon atom forms with more electronegative atoms like oxygen. Therefore in the above sequence: alkane hydrocarbons are level zero (carbon and hydrogen have virtually the same electronegativity), alcohols are level 1 (C–OH, as are halogenoalkanes/haloalkanes e.g. C–Cl), carbonyl is level 2 (e.g. C=O in aldehydes/ketones), carboxylic acids are level 3 (C=O and C–OH) and finally the fully oxidised carbon in carbon dioxide is level 4. Note that a functional group level applies to a single carbon atom e.g. the carbon of the functional group, and not the full molecule.

Similarly for the oxidation sequence from ethane to ethanoic acid ...

CH₃CH₃ (–3,–3) = ox'n => CH₃CH₂OH (–3,–1) = ox'n => CH₃CHO (–3,+1) = ox'n => CH₃COOH (–3,+3)

Note the rise of carbon's oxidation state in increments of 2, see oxidation equations for acidified potassium dichromate(VI) reaction with alcohols and aldehydes in section 9.1(a) where the half–cell oxidation equations involve a 2 electron loss from the organic molecule.

Other organic molecules and redox sequences can be similarly 'analysed'

ethene H₂C=CH₂ (–2,–2) + H₂ (0) == reduction/Ni ==> ethane CH₃–CH₃ (–3,–3), (+1)

propene CH₃–CH=CH₂ (–3,–1,–2) + H₂ == reduction/Ni ==> CH₃–CH₂–CH₃ (–3,–2,–3)

ethanol CH₃–CH₂–OH (–3,–1) == ox'n ==> ethanal CH₃CHO (–3,+1)  == ox'n ==> CH₃COOH (–3,+3)

Appendix 1. The Concept of Functional Group Level

• This concept of functional group level has been introduced into some UK A level pre–university courses.

• It can be related to an increasingly oxidation state of carbon.

• In the explanation with examples below assume R = a H, alkyl or aryl grouping.

• The concept of functional group level of a carbon atom is derived from counting the number of bonds form an individual carbon atom to electronegative atoms (C–X, where I assume X more electronegative than carbon i.e. oxygen, nitrogen or halogens such as chlorine).

• If no such C–X bonds exist the carbon is described as being at the hydrocarbon level.

• With one C–X bond the carbon atom is at the alcohol level e.g. alcohols (C–OH), monohaloalkanes (R₃C–Cl).

• With two C–X bonds the carbon atom is at the carbonyl level e.g. aldehydes and ketones (R₂C=O), ethers (R₃C–O–CR₃)

• With three C–X bonds the carbon atom is at the carboxylic acid level RCOOH (RO–C=O grouping), esters RCOOR, amides RCONH2, nitrile RCN (RN)

• Finally with four C–X bonds we reach the carbon dioxide level e.g. carbon dioxide itself CO₂ or an ether such as C(OR)₄

• The concept of functional group level often applies to a single carbon atom, not the whole molecule e.g.

  • alcohol COH, chloroalkane C–Cl, aldehyde CHO,

  • carboxylic acid level: carboxylic acid COOH, amide CONH₂,

  • but not exclusively e.g. C=C in alkenes, but would this would be considered at the hydrocarbon level despite being at a higher oxidation state than a saturated alkane? (see section 11).

• If a reaction takes place within a level you are swapping one heteroatom (non–carbon atom) for another

  • e.g. hydrolysis of a halogenoalkane: R₃C–Cl + NaOH ==> R₃COH + NaCl

  • which is occurring at the alcohol level

• In order to move a carbon atom up a level requires an oxidizing agent (dealt with on this page in section 9).

  • e.g. the oxidation of alcohols using acidified potassium dichromate(VI) solution

  • R–CH2OH == Cr₂O₇^2–/H⁺ ==> RCHO == Cr₂O₇^2–/H⁺ ==> RCOOH

  • alcohol level ==> carbonyl level ==> carboxylic acid level

• To move a carbon atom down a level requires a reducing agent (dealt with in section 8) or carbanion equivalent (not dealt with yet)

  • e.g. the reduction of carboxylic acids to primary alcohols with lithium tetrahydridoaluminate(III)

  • RCOOH + 4[H] ==> RCH₂OH + H₂O

  • In this case the carbon atom of the functional group is reduced two levels i.e. from the carboxylic level to the alcohol level.

• –

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